A glass preform for use in the fabrication of an optical fiber is produced by various methods. Among them, the VAD method and the OVPD method are attractive methods since their productivity and quality of the fabricated optical fiber are better than other methods. These methods comprise synthesizing glass soot particles by flame hydrolysis of a glass-forming raw material and depositing the soot particles on a rotating seed material to form a porous soot preform. Then, the porous soot preform is heated in a suitable atmosphere containing various components to dehydrate and vitrify it sc as to produce a transparent glass preform, which is drawn to fabricate an optical fiber.
The optical fiber comprises a core through which light is propagated and a cladding which surrounds the core and reflects light to be propagated. Numerical aperture (hereinafter referred to as "N.A.") is calculated from the average refractive indexes n.sub.1 and n.sub.2 of the core and the cladding as follows: ##EQU1## It is understood that the difference of the refractive index between the core and the cladding is made large to obtain a large N.A. To this end, one of following measures is taken in the case of a silica (SiO.sub.2) glass type optical fiber:
1) In the core, an additive for increasing its refractive index is added.
2) In the cladding, an additive for lowering its refractive index is added.
3) Combination of the measures 1 and 2. Needless to say, the cladding in the case 1 and the core in the case 2 are made of silica glass.
Usually, GeO.sub.2, P.sub.2 O.sub.5, Al.sub.2 O.sub.3 and TiO.sub.2 are used to increase the refractive index of silica glass, and B.sub.2 O.sub.3 and fluorine (F) are used to lower the refractive index of silica glass. FIG. 1 shows the variation of the refractive index of silica glass added with the various additives for light with a wavelength of 0.59 .mu.m (cf. Kumamaru and Kurosaki, "Light Transmission Materials" in the Industrial Materials (Kogyozairyo), 27 (1979) 39).
Among the additives, fluorine has recently gained the attention of those skilled persons in this field and is being studied as an additive in the VAD method and the like.
To achieve the same difference of refractive index between the core and the cladding of the optical fiber, the measure 2 or 3 is preferred since any additive is not added to the core, or a smaller amount of the additive is added to the core than in the measure 1. This is advantageous for a high N.A. optical fiber since attenuation of light transmission due to the presence of the additive is decreased. In addition, an optical fiber having good performance in the presence of radiation can be produced only by the measure 2. Therefore, it is greatly advantageous to add the additive to the cladding so as to lower its refractive index.
In the VAD method, fluorine is added in a sintering step, namely a vitrifying step of the soot preform. This has the following advantages:
1. Fluorine is homogeneously added to achieve uniform distribution of the refractive index, and
2. The addition rate of fluorine is high. Namely, several to tens of hundreds of grams of the porous soot preform can be treated and vitrified within several hours.
In the conventional methods, even when the soot preform is heated under atmospheric pressure in an atmosphere of a 100% pure fluorine-containing compound in order to add fluorine to the preform, fluorine is added at most in an amount corresponding to -0.75% of the refractive index difference.
By other methods, for example, a plasma outside deposition method, the glass-forming raw material is deposited on the seed material by means of flame generated by plasma so as to directly vitrify the raw material on the seed material. If the fluorine-containing compound is added in the glass-forming material to add fluorine in the glass in an amount corresponding to -1% of the refractive index difference, the deposition rate of the glass is at most 0.1 g/min. and lowered as the amount of the additive is increased.
However, according to the conventional methods, including the VAD method and the plasma method, bubbles tend to undesirably remain in the glass preform when fluorine is added in an amount corresponding to -0.5% of the refractive index difference. The amount of the bubbles increases as the added amount of fluorine increases.